The present invention relates generally to exhaust systems for marine jet propulsion units, and specifically to exhaust systems for relatively higher-powered, inboard mounted marine jet propulsion units having indirect drive systems, and preferably designed for installation in multi-passenger watercraft.
Conventional marine jet propulsion units are designed to be used instead of propeller-driven outboard or inboard marine motors. Some of the more significant advantages of jet propulsion units include the lack of a depending gear case, which allows the craft to have minimum contact with the water surface at high speed. This feature of jet propulsion units enables the operator to make tight turns while maintaining the boat in a generally horizontal orientation. Another feature of marine jet propulsion units is that the lack of a depending propeller enables the craft to be operated in shallower water without fouling.
An important design consideration of exhaust systems for marine jet propulsion units is that when a watercraft equipped with a jet propulsion unit is under way, the stern of the craft will be elevated from the water surface. The degree of elevation depends in part on the speed of the craft. While at low speed, the engine exhaust is emitted into the water, as speed increases, engine exhaust is emitted into the air, creating a noise suppression problem which requires attenuation of a wide range of frequencies. This is in contrast to conventional outboard marine engines, wherein at high speed, the exhaust is emitted directly into the water.
Another design consideration of marine jet propulsion units is that ambient water is used to cool the exhaust system, and the cooling water is returned to the ambient body of water simultaneously with the emission of the exhaust from the propulsion unit. Although elevated at the stern while under way, once the boat equipped with such a propulsion unit slows down, the stern submerges, and water has the tendency to migrate back into the engine through the exhaust system if not arrested in some way.
Still another design factor of marine jet propulsion exhaust systems is that a wide range of frequencies must be attenuated to provide sufficient noise reduction for pleasant boating, while not restricting the exhaust gas flow from the engine. Conventional marine muffler technology has been found to be unsatisfactory in attenuating all target frequencies in jet propulsion units without unduly restricting engine exhaust gas flow.
Accordingly, a first object of the present invention is to provide an exhaust system for a marine jet pump unit which provides sufficient sound attenuation without detracting from the flow of engine exhaust gases.
Another object of the invention is to provide an exhaust system for a marine jet pump unit which protects the engine from the inflow of ambient water, especially while the stern of the boat powered by the jet pump is submerged.
A further object of the present invention is to provide an exhaust system for a marine jet pump which may be tailored to attenuate specific target frequencies.
Yet another object of the present invention is to provide an exhaust system for a marine jet pump which cools the exhaust prior to emission into the ambient environment.